Let sleeping dogs lies is an idiom that has existed since at least the 1300s, with cognates in several languages, including German and French.

That you should 'let sleeping dogs lie,'
is an old concept.

Meaning

When we say "let sleeping dogs lie" we mean to not bring up problems that will themselves bring up even bigger problems. In other words, we are saying to not stir things up.

That you should 'let sleeping dogs lie,'
is an old concept.

This is such an important concept, that some problems are better left unmentioned, that there are several methophorical expressions which mean the same thing, such as leave well enough alone, (don't) open a can of worms, and never trouble trouble till trouble troubles you. This last gives a glimpse into the metaphor: If a 'problem' isn't causing any trouble for you, why bring it up? Just like when you wake a sleeping dog, you might get bit.